So, I'm not really sure how to even start on this word problem. Could someone help and walk me through it? Thanks a lot.

The number of inches that a young redwood tree grows per year can be modeled by f(x)=−0.05x2+x+1, where x represents annual rainfall in inches, and f(x) is the tree's annual growth, in inches. How many inches of rainfall produces the maximum annual growth in the tree?